The present invention relates generally to a heat sink, a circuit board, and an electronic apparatus, and more particularly to a fixture of a heat sink onto the circuit board mounted with an exoergic circuit element (simply referred to as an exoergic element). The “electronic apparatus,” as used herein, intends to cover, for example, a laptop personal computer (“PC”), a personal digital assistant (“PDA”), a server, an electronic dictionary, electronic stationery, and a game machine.
Along with the recent widespread electronic apparatuses, smaller and higher-performance electronic apparatuses have been increasingly demanded, and a reduction of the number of components is studied. A laptop PC is one typical electronic apparatus mounted with an exoergic element, such as a CPU and a chipset, and the heating value of the exoergic element increases as its performance improves. In order to thermally protect the exoergic element, a radiator referred to as a heat sink is thermally connected to the exoergic element. The heat sink includes one or more cooling fins, and radiates the exoergic element through natural cooling. The heat sink is placed on the exoergic element, and clamped at four corners around the exoergic element via fixture members. Each fixture member, such as a bolt that perforates a coil spring, presses the heat sink against the exoergic element, reduces the heat transmission loss, and maintains radiation efficiency.
Conventionally, a heat sink is mounted for each exoergic element, or no heat sink is mounted if the heating value of the exoergic element is lower than a preset one. One proposed method uses one radiator to simultaneously radiate plural exoergic elements, since the recent increasing mounting density arranges the CPU closer to the chipset. See, for example, Japanese Patent Application Nos. 08-255856, 07-058470, and 08-023182.
However, these references are silent about the way of fixing one heat sink onto plural exoergic elements. Even for one exoergic element, when the heat sink is compressively clamped onto the exoergic element at four corners, the compressive force does not become uniform. The heat transmission loss and thus the poor radiation effect are conspicuous as the compressive force decreases, causing a thermal breakdown. It is conceivable to increase the compressive force as a whole so that the minimum compressive force can exceed a preset value, but this configuration causes an overload and thus a mechanical breakdown at a highest pressure portion. As such, an elastic member, such as silicon rubber, may be inserted between the heat sink and the exoergic element so that the elastic member can rectify an uneven pressure distribution. However, the elastic member has such low heat conductivity that the radiation efficiency of the exoergic element deteriorates. As the recent exoergic element increases its heating value, the elastic member should be made thinner and it is difficult to maintain the thickness of the elastic member enough to rectify the uneven pressure distribution. Further, in radiating plural exoergic elements with one heat sink, one of them which is insufficiently cooled is subject to thermal breakdown.